The present invention relates to a single longitudinal mode semiconductor laser suitable as a light source for optical fiber communication systems.
As the transmission loss of optical fibers has been drastically reduced to no more than 0.2 to 0.5 dB/km in the 1.3 and 1.5 micron wavelength bands, it has become possible to realize an optical fiber communication system having a relaying distance of more than 100 km. In long-distance transmission, the transmissible relaying distance and capacity are limited not only by the transmission loss of optical fibers but also by wavelength dispersion. The effect of wavelength dispersion is remarkable in long-distance optical fiber transmission using the conventional Fabry Perot reservoir type semiconductor lasers, which usually have a plurality of oscillating longitudinal modes.
Realization of a long-distance large-capacity optical fiber communication system would require semiconductor lasers capable of oscillating in a single longitudinal mode even in high-speed modulation.
Such semiconductor lasers include the distributed feedback laser diode (DFB LD) with a built-in diffraction grating having a periodic structure and the distributed Bragg reflector laser diode (DBR LD). These semiconductor lasers, which can select the oscillation longitudinal mode, are still in the process of research and development, and only recently became capable of continuous operation at room temperature. There is a long way to go before they can be successfully practicable because they are inferior to the conventional Fabry Perot oscillator type semiconductor laser in such basic areas as oscillation threshold and differential quantum efficiency. The DFB LD and the DBR LD are deficient because they have lower equivalent reflecting power than the conventional semiconductor laser. This condition results from the weak coupling between light and the periodic structure constituting the diffraction grating, resulting in a poor light diffraction efficiency. In a DBR LD, for instance, a low reflecting power in the diffraction grating results in an increase in injection current required for laser oscillation.
Moreover, as the region in which the grating is formed has to be made as long as possible in order to increase the reflecting power in the diffraction grating section, the overall element length is inevitably extended to around 1 mm. There are other disadvantages relating to element performance and fabrication.